anemic
For gravel I'm 100% a tubes-with-sealant guy. 

Summary: CX0 700 x 40 worked great for me after a break in period; felt faster than other tires, provided climbing traction. I'd buy CX0 again.

I ride a Trek Crockett (alu frame, carbon fork) CX race bike on gravel in Michigan where we encounter highly variable gravel conditions on our roads; deep loose beach sand conditions, packed clay super fast center-road single track, sharp loose gravel, leaf covered corners, erosion & washboard. We have miles of flats but also sharp short steep kickers from 50 to over 200 vertical feet. Some of our gravel races include seasonal roads which are much better suited to MTB tires, but then they also include some high speed pavement as well, so a drop bar rig is favored. This bike is my primary bike but I greatly enjoy MTB riding & racing as well. 

OE tires were the CX3 which lasted one ride before I swapped them out. They look like a deep grass or even loose dry gravel, or even maybe a medium mud tire (straight knobbies). For these reasons, I perceived more drag at speed than I prefer. Later I installed them and trained on them on grass and found them to be great for their intended purpose. It's a very nice tire but not for gravel.

I had some Stan's Ravens on hand (700 x 35) which I installed. I find these tires to be somewhat draggy despite a fast medium-looking tread design. A fast friend recently raced on a pair of these in a gravel race and was unusually slow. I think these may not be a great gravel tire. Just okay for training. They look like they should be faster. 

I feel that both of these tires felt more draggy than my MTB tires (Ralph & Ron at 20 psi) on pavement and even fast gravel so I was unhappy and looking for a solution. 

So I bought one 700 x 40 CX0 (tubes version) to give that a go in the rear for better traction for climbing and increased speed (As a confident descender I'm not concerned about a loose front). It felt a little more draggy than I hoped! I aired it up a lot, 50 psi to try to develop a feel for the upper end and I lost some climbing traction yet I did not seem to gain much speed, but it was serviceable.

But THEN! after an unknown number of miles the center of the tire tread wore down smooth and it came into it's own! Most unexpected. I really like this tire. Now it feels fast and it is fast. It's no longer draggy. It's highly reliable in loose conditions. It grips GREAT in rain softened gravel, loose steep climbs and the side knobs provide adequate traction for occasional muddy low spots like next to marshes, ponds, etc. And it's fast after you burn it in. I value & feel suppleness. 

I plan to buy more CX0 tires. I am considering the 700 x 34. I would like to try it on the rear, and I will put the worn 40 on the front. I run around 45 psi. I have been using the Raven up front where I don't think the draggy feel is as much of an issue as it is in the rear. I weigh 150#. I have a friend with the same frame and wider rims (Ardennes) and he cannot fit these or even 38s in the frame. I have a medium wide rim, Bontrager RXL TLR (guessing 19mm wide). I would not run a brand new CX0 in a race. Break in for at least a hundred miles at higher PSI to achieve the smooth center spot. And then it will be magnificent. 


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BluesDawg
CXO? Brand?
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frontrangegravel
BluesDawg - Bontrager CXO. I can't update his thread name to include that. Going forward, I'll spend a little time populating this so we have brand and model. 
http://www.trekbikes.com/us/en_US/equipment/cycling-components/bike-tires-tubes/bontrager-cx0-cyclocross-tire/p/09090

09090_B_1_CXO.jpg 
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anemic
Nice tire photo frontranger. Thanks. I was able to use the full editor to add the brand name to the thread name. Sorry I didn't originally realize that would be a good idea. 
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frontrangegravel
Cool. Thanks anemic
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lennymatuszewski
I just wanted to add to this topic because I've recently revisited my CX0's. Currently I'm running the 34mm non-tubeless version because I need new Stan's tape and these tires need to get used up. I'm coming off of Clement XPlor USH 35mm's, which I wasn't all too impressed with. I forgot how fast rolling the CX0's can be on pavement, especially when that center tread wears down. Knock on wood, I still haven't had a puncture on gravel yet. They do seem like they're a little too fragile in the sidewall for serious gravel grinding, but they've been holding up fantastically for me! They're extremely fast on the hardpack and supple enough to not wash out in the messy stuff. I'm currently running them at 40/45 PSI. I've got a brand new set of 33mm tubeless waiting to go on once these have burned out. I expect to like them even more.
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Fett
The Bontrager CX0 in the 38mm size has been my go to gravel tire for several years now.  I echo the post that if you wear down the center a bit, it is very fast ad. Still has great cornering grip. I live in Indiana and have podiumed several Gravel Grovels and Death Marchs with them. These are races that will take you on the full range of conditions with some singletrack, gravel, pavement and rugged double track and it has been great. I used these on the Red River Gorge 100 on my mountain bike as it was gravel heavy and was a great choice 
  
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